Solar energy reflector

ABSTRACT

Solar energy reflectors ( 1 ) according to the present invention comprise a mirror ( 5 ) laminated to a support ( 6 ) by means of a bonding material ( 9 ) comprising a foam tape.

This invention relates to solar energy reflectors and to processes fortheir manufacture.

The reflectors of this invention may be used in solar energy or heatinginstallations, for example concentrating solar power plants. Suchinstallations use the solar energy to first generate heat, which latercan be converted into electricity or used for steam production.Concentrating solar power plants wherein reflectors according to thepresent invention may be used comprise, for example, parabolic troughpower plants, central tower power plants (also called heliostat powerplants), dish collectors and Fresnel reflector power plants. Solarenergy reflectors according to the present invention may be used in suchinstallations as flat or curved mirrors.

Solar energy reflectors may be produced by forming a laminate comprisinga thin mirror bonded to a supporting sheet having substantially the samesurface dimensions as the mirror. Maximum reflectivity for the mirrormay be obtained if it is thin, so that less solar energy is absorbedwhen passing through the substrate of the mirror. However thin mirrorsmay be poor in terms of mechanical resistance, therefore it may benecessary to laminate them on a supporting substrate, for example ametallic sheet. The mirror and the supporting substrate then form asingle laminated structure. Alternatively, solar energy reflectors maybe produced by using a mirror adapted to offer enough mechanicalresistance for the mirror to stand without a supporting sheet, e.g. amirror having a greater thickness. Supporting means may be attached tothe mirror for fixation in the solar energy installation and/or may helpin maintaining the curved shape of the mirror.

It is known to laminate mirrors to their support to provide a laminatedassembly, by means of epoxy resins, silicone-based adhesives,polyurethane adhesives, hot-melt adhesives, acrylic resin based adhesiveor a polyvinylchloride bonding layer. It is generally found advantageousfor the bonding material to comprise an acrylic resin. Epoxy, siliconeand polyurethane resins, in certain circumstances, may release chemicalspecies which may attack a paint layer of the mirror and finally corrodethe silver layer; epoxy resins may create tensile stress between thepaint and silver layers of the mirror when cross-linking takes place,which may cause detachments to occur in the mirror structure; hot-meltadhesives may lose at least part of their elasticity when exposed tohigher temperatures, which may cause the mirror to detach from thesupporting sheet; some silicone based materials (e.g. structuralsilicone) may be too rigid when the metallic sheet dilates whensubjected to differences in operating temperatures, which may cause thepaint and silver layers of the mirror to be pulled out; silicone maycreate planarity defects during lamination; polyurethane resins may notbe sufficiently resistant to UV rays. Acrylic resins may showadvantageous properties in terms of chemical neutrality, resistance toUV rays, and flexibility and elasticity. The bonding material mayalternatively comprise EVA (ethylvinylacetate) or other acetate-basedpolymer film, PVB (polyvinylbutyral), TPU (thermoplastic urethane) orionomer-based films. Such materials may show advantageous properties interms of chemical neutrality, resistance to UV rays, and flexibility andelasticity. Other known bonding materials may comprise an acrylicpressure-sensitive adhesive. This may be provided as a transfer tapewherein the adhesive is present between two supporting release sheetswhich are intended to be removed. Alternatively, it may be provided as asupported tape wherein the adhesive is provided on both side of asupporting sheet which is intended to stay. Such a supported tape may,for example, comprise a core which consists essentially of a PET film ora polyester foil, surrounded by layers consisting essentially of theacrylic pressure-sensitive resin.

According to one of its aspects, the present invention provides a solarenergy reflector as defined by claim 1. Other claims define preferredand/or alternative aspects of the invention.

Solar energy reflectors according to the invention comprise a mirrorlaminated to a support by means of a bonding material comprising a foamtape, so that the mirror and support form a laminated assembly, i.e. asingle laminated structure constructed in such a way that the bondingmaterial units together the mirror and the support. Preferably thebonding material consists essentially of the foam tape.

This may provide advantageous properties including one or anycombination of:

-   -   adhesion directly after application may be greater than with an        identical adhesive not supported by a foam carrier;    -   thickness and flexibility of the foam tape may provide an        effective bonding surface which is greater than for example a        typical thin and more rigid supported tape of PET with acrylic        resin. Indeed, a thin and more rigid bonding material, contrary        to foam, may not adapt to irregularities of planarity in the        mirror and/or the support and consequently may not adhere        correctly over its whole surface, creating air bubbles trapped        between the mirror and/or the support and the bonding material.        These defects are known as “popping”; they may create optical        defects in the reflector. The present invention may reduce or        avoid the risk of popping;    -   elasticity of the foam tape may allow for slight relative        parallel displacement of the bonded faces of the mirror and        support, for example in consequence of flexing of the laminate        or differential thermal expansion of the mirror and the support;    -   adequate bond strength and high degree of water or moisture        resistance of the foam tape may provide good corrosion        resistance to the reflector;    -   good resistance to UV rays.

The foam tape may advantageously comprise, or preferably consistessentially of, a foam carrier coated on its two main surfaces with anadhesive, i.e. a foam core surrounded by layers of adhesive material.

Preferably, the foam carrier comprises, or more preferably consistsessentially of, at least one material selected from the group consistingof polyethylene, urethane, vinyl, neoprene, EPDM and polyester.Polyethylene may be preferred for its low cost. We have found that, morethan the adhesive coated on the foam carrier, it seems to be themechanical resistance of the foam itself which determines the resistanceof the reflector against delamination. Cohesion of the reflector may becontrolled by selecting appropriate foam carriers. Advantageously, thefoam carrier may consist essentially of polyethylene and may have atensile strength according to DIN 53455 (measured on a 15 mm broadsample) of at least 5, preferably at least 8 or more preferably at least10 N/mm²; its tensile strength may be less than or equal to 50,preferably less than or equal to 40, more preferably less than or equalto 30, or still more preferably less than or equal to 25 N/mm². It maybe advantageous for the polyethylene foam carrier to have an elongationaccording to DIN 53455 of at least 80, preferably at least 100, morepreferably at least 200, or still more preferably at least 240%;elongation may be less than or equal to 500, preferably less than orequal to 450 or more preferably less than or equal to 400%. The foamcarrier may have a thickness of at least 0.2, preferably at least 0.4mm; thickness may be less than or equal to 3.5 mm, preferably less thanor equal to 2 mm.

Advantageously, the foam carrier consists essentially of a closed cellfoam. This may help provide good water, water vapour and moisturesealing and consequently help provide good durability to the reflector.

Preferably, the adhesive coating the foam carrier comprisesacrylic-based or structural adhesives; it may comprise, or morepreferably consist essentially of, at least one material selected fromthe group consisting of acrylic, silicone, polyurethane, epoxy, MSpolymer and rubber. It may be preferred for the adhesive to consistessentially of acrylic. Acrylic resins may show advantageous propertiesin terms of chemical neutrality, resistance to UV rays, and flexibilityand elasticity. They may further offer a rapid adhesive tack. Inparticular embodiments, the adhesive may be different on both sides ofthe foam carrier.

In preferred embodiments of the invention, the bonding material ispresent on substantially the whole surface of the support facing themirror. This may provide good adhesion and resistance to delaminationand/or may reduce the risk of water entering the space between thesupport and the mirror, which could increase the risk of corrosion ofthe mirror.

Prior art mirrors used in solar energy reflectors have generally beenproduced as conventional domestic mirrors used for interiorapplications, i.e. as follows: a sheet of flat glass (float, soda-limeglass) was first of all polished and then sensitised, typically using anaqueous solution of SnCl₂; after rinsing, the surface of the glass wasusually activated by means of an ammoniacal silver nitrate treatment,and a silvering solution was then applied in order to form an opaquecoating of silver; this silver coating was then covered with aprotective layer of copper and then with one or more coats of leadedpaint in order to produce the finished mirror. The combination of theprotective copper layer and the leaded paint was deemed necessary toprovide acceptable ageing characteristics and sufficient corrosionresistance.

More recently, mirrors were developed which dispensed with the need forthe conventional copper layer, which could use substantially lead-freepaints and yet which still had acceptable or even improved ageingcharacteristics and corrosion resistance. For example, U.S. Pat. No.6,565,217 describes embodiments of a mirror with no copper layer whichcomprises in the order recited: a vitreous substrate; both tin andpalladium provided at a surface of the vitreous substrate; a silvercoating layer on said surface of the substrate; tin present at thesurface of the silver coating layer which is adjacent to an at least onepaint layer; and at least one paint layer covering the silver coatinglayer. Such mirrors provided a significant advance with respect toconventional coppered mirrors.

Solar energy reflectors according to the present invention preferablycomprise a mirror which is free of a copper layer and comprises a glasssubstrate, a silver coating layer provided at a surface of the glasssubstrate and at least one paint layer covering the silver coatinglayer. Preferably, the mirror is laminated to the support so that the atleast one paint layer is facing the support. The silver coating layerprovides the reflective layer of the mirror (which reflects the sun raysthat pass through the glass sheet). The at least one paint layer mayprovide a protection for the silver layer from possible chemical attacksby the bonding material, and a surface to which the bonding material canadhere.

Solar energy reflectors according to the invention may comprise an edgeprotection provided on the edges of the mirror. This may help protectthe exposed edges of the silver layer against corrosion.

The support may consist essentially of at least one material selectedfrom the group consisting of metallic materials, plastic materials,composite materials and glass. Preferably, it is made of steel,stainless steel, galvanised steel, painted steel, or aluminium.

In preferred embodiments, the support is a sheet having substantiallythe same surface dimensions, i.e. same length and same width, as themirror. This includes embodiments wherein the support may be slightlygreater in size than the mirror, and the mirror may be bonded to thesupport such that projecting margins, of for example 5 mm, may extendbeyond the periphery of the mirror.

Preferably, the thickness of the support, when it is a metallic sheet,may be greater than 0.5 mm or 0.6 mm and less than 1 mm or 0.9 mm; itmay preferably be around 0.7 or 0.8 mm.

Advantageously, the thickness of the mirror, when it is laminated on itswhole surface to the support, may be greater than 0.9 mm or 1.1 mm; itmay be less than 2 mm or 1.5 mm; it may preferably be around 0.95 or1.25 mm. Such thin and flexible mirrors may be used in applications werecurved reflectors are needed. Curved reflectors may also be manufacturedwith thicker mirrors which are not laminated to a support on their wholesurface; in that case, the thickness of the mirrors may be greater than2 mm or 2.5 mm; it may be less than 5 mm or 4.5 mm. When flat reflectorsare used, the total thickness of the mirror may be greater than 2 mm or2.5 mm; it may be less than 6 mm or 5 mm.

Preferably, the silver coating layer of the mirror has a thickness of atleast 80 nm, at least 100 nm, more preferably at least 120 nm, or atleast 140 nm; its thickness may be less than 200 nm, preferably lessthan 180 nm. The layer of silver may contain between 800 and 2000 mg/m²of silver, preferably between 1400 and 1800 mg/m² of silver. Thesevalues offer a good compromise between a good energetic reflectancevalue for the reflector and an acceptable cost of production.Preferably, the glass substrate of the mirror is made of extra-clearglass, i.e. a glass with a total iron content expressed as Fe₂O₃ of lessthan 0.02% by weight. This also may favour a good energetic reflectancevalue for the reflector.

In one preferred embodiment of mirrors for solar energy reflectorsaccording to the invention, the paint layer or at least one of the paintlayers applied over the silver layer is lead-free or substantiallylead-free. This is advantageous in that lead is toxic and its avoidancehas environmental benefits. Substantially lead-free means herein thatthe proportion of lead in the paint is significantly less than theproportion of lead in leaded paints conventionally used for mirrors. Theproportion of lead in a substantially lead-free paint layer as hereindefined is less than 500 mg/m², preferably less than 400 mg/m², morepreferably less than 300 mg/m². The proportion of lead in a lead-freepaint layer as herein defined is less than 100 mg/m², preferably lessthan 80 mg/m², more preferably less than 60 mg/m².

The finished reflector may have an energetic reflectance according tostandard ISO 9050:2003 of greater than 90%, preferably greater than 92%.The energetic reflectance may be less than 97% or less than 96%.

Embodiments of the invention will now be further described, by way ofexample only, with reference to FIGS. 1 to 2 and to examples 1 to 5,along with comparative examples 1 to 4.

FIG. 1 is a schematic cross-section of a solar energy reflectoraccording to the invention.

FIG. 2 is a schematic view of a curved solar energy reflector accordingto the invention.

FIG. 3 is a schematic view of a dynamic shear test. Figures are notdrawn to scale.

FIG. 1 shows a solar energy reflector (1) which comprises a mirror (5)laminated to a support in the form of a sheet (6), for example of metal,by means of a bonding material (9) consisting essentially of a foamtape. The mirror comprises a glass substrate (2), a silver layer (3) andat least one paint layer (4). The foam tape (9) comprises a foam carrier(7) coated on its two main surfaces with an adhesive (8, 8′).

FIG. 2 shows a curved solar energy reflector (10) which comprises amirror (5) laminated to a support in the form of two distinct profiles(60, 60′), for example of metal, by means of foam tapes (90, 90′).

FIG. 3 shows a sample comprising a mirror (5), a tape (99) and a steelsupport (6) subjected to a dynamic shear test. The arrows showdirections of movement.

Examples 1 to 4 and comparative examples 1 to 3 (not in accordance withthe present invention) report adhesion tests measurements made onbonding materials adhered to mirrors of the type MNGE®, i.e. mirrorswith no copper layer commercialised by AGC Flat Glass Europe SA. Resultsare given in Tables I and II. AF means adhesive failure and CF meanscohesive failure.

In example 1, the bonding material is a foam tape having a polyethylenefoam carrier of 0.8 mm thickness coated with a pure acrylic adhesive.Example 2 has the same bonding material as example 1 except that thefoam carrier has a thickness of 1.6 mm. Example 3 has the same bondingmaterial as example 1 except that the foam carrier has a thickness of 2mm. Example 4 has the same bonding material as example 1 except that thefoam carrier has a thickness of 3.2 mm.

In comparative example 1, the bonding material is an acrylic transfertape; the acrylic adhesive of this transfer tape is identical to theacrylic adhesive of examples 1-4. In comparative example 2, the bondingmaterial is an acrylic supported tape of 75 μm thickness comprising aPET support of 12 μm thickness; the acrylic adhesive of this supportedtape is of a lower quality in terms of adhesion than the acrylicadhesive of examples 1-4. In comparative example 3, the bonding materialis an acrylic supported tape of 130 μm thickness, comprising a PETsupport of 12 μm thickness; the acrylic adhesive of this supported tapeis identical to the acrylic adhesive of examples 1-4.

The peel test measures the strength required to pull apart a bondedsurface. Peel test measurements were made on mirrors at 180° after 20minutes and 24 hours without load. Results of these tests (Table I) showbest results for foam tapes and for an acrylic supported tape (comp. ex.3).

The static shear test shows the ability of the bonding material towithstand a fixed load over time. For these tests, adhesives were placedon the mirror and left during 1 hour at 23° C. for some samples and 80°C. for others; then a weight of 1 kg was fixed to the sample. Theadhesive surface was 12.5×12.5 mm. Best results (see Table I) areobtained for foam tapes, especially at room temperature. At highertemperature, thinner foam tapes seem to give better results. Comparativeexamples show that acrylic transfer or supported tapes give inferiorresults.

TABLE I thickness Peel test Peel test Static shear Static shear ofbonding 20 min 24 h test test Example N° bonding material material [N/25mm] [N/25 mm] 80° C. 23° C. Comp. ex. 1 acrylic transfer tape 78 μm 13.7AF 16.3 AF 15 min AF 40 min AF Comp. ex. 2 acrylic supported tape 75 μm9.9 AF 12.5 AF 10 min AF 0 min AF Comp. ex. 3 acrylic supported tape 130μm 18.8 AF 22 AF 15 min AF 15 min AF 1 foam tape 0.9 mm 19.6 AF 24.2 AF1 h 10 CF 6-22 h AF 2 foam tape 1.7 mm 23 AF 31.9 CF 1 h 30-2 h CF 8-22h AF 3 foam tape 2.1 mm 18.1 AF 29.6 AF 30 min CF 31-46 h AF 4 foam tape3.3 mm 15 AF 35 AF 20 min CF 4 h 30 AF

The dynamic shear test was realised as follows (see FIG. 3). A sample ofmirror (2.5×10×0.4 cm) and another of painted galvanised steel (2.5×6.5cm) were cleaned and dried. A piece of tape of 1.5×2.5 cm was applied onthe mirror and pressed with a roller of 1 kg. The steel sample was thenapplied on the tape and pressed with the roller. The assembly was thenleft to polymerise during 2 days at room temperature and humidity.Initial values were measured after these 2 days and other measurementswere made after 1 week of ageing at room temperature. Shear forces wereapplied to the samples until failure, with a traction speed of 5 mm/minand at room temperature. Three or four measurements were taken for eachexample and comparative example and mean values and standard deviationsof these results are given in Table II. This shows the advantage of thefoam tapes over an acrylic supported tape and especially thereproducibility of the results for the foam tapes (lower standarddeviations).

TABLE II Comp. ex. 2 Ex. 1 Ex. 2 Dynamic shear displacement mean 0.985.07 6.23 test max load [mm] st. dev. 0.24 0.35 0.23 after 2 days maxstress mean 2.39 4.80 3.50 [kg/cm²] st. dev. 1.34 0.44 0.10 Dynamicshear displacement mean 0.90 5.17 6.70 test max load [mm] st. dev. 0.330.21 0.26 after 1 week max stress mean 2.93 5.33 3.87 [kg/cm²] st. dev.3.68 0.38 0.23In example 5, a mirror of 40 cm×60 cm is laminated to a supporting sheetof steel having substantially the same surface dimensions as the mirror,with a foam tape having a polyethylene foam carrier coated with acrylicadhesive. The assembly is curved. It is placed in a static oven at aconstant temperature of 80° C. during 1 month. After this treatment, nohaze is visible on the mirror and no peeling-off or detachment isobserved. In comparative example 4, the same assembly and test are madeexcept that the bonding material is an acrylic transfer tape. Haze isvisible on the whole surface of the mirror, in particular on the edgeportions thereof, and peeling-off of the mirror silver layer is observedon the edges.

1. A solar energy reflector comprising a mirror laminated to a supportwith a bonding material, wherein the bonding material comprises a foamtape.
 2. The solar energy reflector according to claim 1, wherein thefoam tape comprises a foam carrier coated on its two main surfaces withan adhesive.
 3. The solar energy reflector according to claim 2, whereinthe foam carrier consists essentially of at least one material selectedfrom the group consisting of polyethylene, urethane, vinyl, neoprene,EPDM and polyester.
 4. The solar energy reflector according to claim 2,wherein the foam carrier consists essentially of a closed cell foam. 5.The solar energy reflector according to claim 2, wherein the adhesivecomprises at least one material selected from the group consisting ofacrylic, silicone, polyurethane, epoxy, MS polymer and rubber.
 6. Thesolar energy reflector according to claim 2, wherein the foam carrierconsists essentially of polyethylene and has a tensile strengthaccording to DIN 53455 within the range of from 5 to 50 N/mm².
 7. Thesolar energy reflector according to claim 2, wherein the foam carrierconsists essentially of polyethylene and has an elongation according toDIN 53455 within the range of from 80 to 500%.
 8. The solar energyreflector according to claim 2, wherein the foam carrier has a thicknesswithin the range of from 0.2 to 3.5 mm.
 9. The solar energy reflectoraccording to claim 1, wherein the bonding material is present onsubstantially the whole surface of the support facing the mirror. 10.The solar energy reflector according to claim 1, wherein the mirror isfree of a copper layer and comprises a glass substrate, a silver coatinglayer provided at a surface of the glass substrate and at least onepaint layer covering the silver coating layer.
 11. The solar energyreflector according to claim 1, wherein the mirror is laminated to thesupport so that the at least one paint layer is facing the support. 12.The solar energy reflector according to claim 1, wherein the supportconsists essentially of at least one material selected from the groupconsisting of a metallic material, a plastic material, a compositematerial and glass.
 13. The solar energy reflector according to claim 1,wherein the support is a sheet having substantially same surfacedimensions as the mirror.
 14. (canceled)